The present invention relates to a method of manufacturing an impeller element having a shroud attached to the impeller blades, and particularly to a compressor/turbine permanent magnet motor/generator having a shroud attached to the impeller blades by an interference fit therebetween.
A helical flow compressor is a high-speed rotary machine that accomplishes compression by imparting a velocity head to each fluid particle as it passes through the machine""s impeller blades and then converting that velocity head into a pressure head in a stator channel that functions as a vaneless diffuser. While in this respect a helical flow compressor has some characteristics in common with a centrifugal compressor, the primary flow in a helical flow compressor is peripheral and asymmetrical, while in a centrifugal compressor, the primary flow is radial and symmetrical. The fluid particles passing through a helical flow compressor travel around the periphery of the helical flow compressor impeller within a generally horseshoe shaped stator channel. Within this channel, the fluid particles travel along helical streamlines, the centerline of the helix coinciding with the center of the curved stator channel. This flow pattern causes each fluid particle to pass through the impeller blades or buckets many times while the fluid particles are traveling through the helical flow compressor, each time acquiring kinetic energy. After each pass through the impeller blades, the fluid particles reenter the adjacent stator channel where they convert their kinetic energy into potential energy and a resulting peripheral pressure gradient in the stator channel.
The multiple passes through the impeller blades (regenerative flow pattern) allows a helical flow compressor to produce discharge heads of up to fifteen (15) times those produced by a centrifugal compressor operating at equal tip speeds. Since the cross-sectional area of the peripheral flow in a helical flow compressor is usually smaller than the cross-sectional area of the radial flow in a centrifugal compressor, a helical flow compressor would normally operate at flows which are lower than the flows of a centrifugal compressor having an equal impeller diameter and operating at an equal tip speed. These high-head, low-flow characteristics of a helical flow compressor make it well suited to a number of applications where a reciprocating compressor, a rotary displacement compressor, or a low specific-speed centrifugal compressor would not be as well suited.
A helical flow compressor can be utilized as a turbine by supplying it with a high pressure working fluid, dropping fluid pressure through the machine, and extracting the resulting shaft horsepower with a generator. Hence the term xe2x80x9ccompressor/turbinexe2x80x9d which is used throughout this application.
Among the advantages of a helical flow compressor or a helical flow turbine are:
(a) simple, reliable design with only one rotating assembly;
(b) stable, surge-free operation over a wide range of operating conditions (i.e. from full flow to no flow);
(c) long life (e.g., 40,000 hours) limited mainly by their bearings;
(d) freedom from wear product and oil contamination since there are no rubbing or lubricated surfaces utilized;
(e) fewer stages required when compared to a centrifugal compressor; and
(f) higher operating efficiencies when compared to a very low specific-speed (high head pressure, low impeller speed, low flow) centrifugal compressor.
On the other hand, a helical flow compressor or turbine cannot compete with a moderate to high specific-speed centrifugal compressor, in view of their relative efficiencies. While the best efficiency of a centrifugal compressor at a high specific-speed operating condition would be on the order of seventy-eight percent (78%), at a low specific-speed operating condition of a centrifugal compressor could have an efficiency of less than twenty percent (20%). A helical flow compressor operating at the same low specific-speed and at its best flow can have efficiencies of about fifty-five percent (55%) with curved blades and can have efficiencies of about thirty-eight percent (38%) with straight radial blades.
The flow in a helical flow compressor can be visualized as two fluid streams which first merge and then divide as they pass through the compressor. One fluid stream travels within the impeller buckets and endlessly circles the compressor. The second fluid stream enters the compressor radially through the inlet port and then moves into the horseshoe shaped stator channel which is adjacent to the impeller buckets. Here the fluids in the two streams merge and mix. The stator channel and impeller bucket streams continue to exchange fluid while the stator channel fluid stream is drawn around the compressor by the impeller motion. When the stator channel fluid stream has traveled around most of the compressor periphery, its further circular travel is blocked by the stripper plate. The stator channel fluid stream then turns radially outward and exits from the compressor through the discharge port. The remaining impeller bucket fluid stream passes through the stripper plate within the buckets and merges with the fluid just entering the compressor/turbine.
The fluid in the impeller buckets of a helical flow compressor travels around the compressor at a peripheral velocity which is essentially equal to the impeller blade velocity. It thus experiences a strong centrifugal force which tends to drive it radially outward, out of the buckets. The fluid in the adjacent stator channel travels at an average peripheral velocity of between five (5) and ninety-nine (99) percent of the impeller blade velocity depending upon the compressor discharge flow. It thus experiences a centrifugal force which is much less than that experienced by the fluid in the impeller buckets. Since these two centrifugal forces oppose each other and are unequal, the fluid occupying the impeller buckets and the stator channel is driven into a circulating or regenerative flow. The fluid in the impeller buckets is driven radially outward and xe2x80x9cupwardxe2x80x9d into the stator channel. The fluid in the stator channel is displaced and forced radially inward and xe2x80x9cdownwardxe2x80x9d into the impeller bucket.
The fluid in the impeller buckets of a helical flow turbine travels around the turbine at a peripheral velocity which is essentially equal to the impeller blade velocity. It thus experiences a strong centrifugal force which would like to drive it radially outward if unopposed by other forces. The fluid in the adjacent stator channel travels at an average peripheral velocity of between one hundred and one percent (101%) and two hundred percent (200%) of the impeller blade velocity, depending upon the compressor discharge flow. It thus experiences a centrifugal force which is much greater than that experienced by the fluid in the impeller buckets. Since these two centrifugal forces oppose each other and are unequal, the fluid occupying the impeller buckets and the stator channel is driven into a circulating or regenerative flow. The fluid in the impeller buckets is driven radially inward and xe2x80x9cupwardxe2x80x9d into the stator channel. The fluid in the stator channel is displaced and forced radially outward and xe2x80x9cdownwardxe2x80x9d into the impeller bucket.
While the fluid is traveling regeneratively, it is also traveling peripherally around the stator-impeller channel. Thus, each fluid particle passing through a helical flow compressor or turbine travels along a helical streamline, the centerline of the helix coinciding with the center of the generally horseshoe shaped stator-impeller channel.
While the unique capabilities of a helical flow compressor would seem to offer many applications, the low flow limitation has severely curtailed their widespread utilization.
Permanent magnet motors and generators, on the other hand, are used widely in many varied applications. This type of motor/generator, such as in U.S. Pat. No. 5,899,673, has a stationary field coil and a rotatable armature of permanent magnets. In recent years, high energy product permanent magnets having significant energy increases have become available. Samarium cobalt permanent magnets having an energy product of twenty-seven (27) megagauss-oersted (mgo) are now readily available and neodymium-iron-boron magnets with an energy product of thirty-five (35) megagauss-oersted are also available. Even further increases of mgo to over 45 megagauss-oersted promise to be available soon. The use of such high energy product permanent magnets permits increasingly smaller machines capable of supplying increasingly higher power outputs. The permanent magnet rotor may comprise a plurality of equally spaced magnetic poles of alternating polarity or may even be a sintered one-piece magnet with radial orientation. The stator would normally include a plurality of windings and magnet poles of alternating polarity. In a generator mode, rotation of the rotor causes the permanent magnets to pass by the stator poles and coils and thereby induces an electric current to flow in each of the coils. In the motor mode, electrical current is passed through the coils which will cause the permanent magnet rotor to rotate.
In various rotating impeller designs, shrouds have been added to improve aerodynamic performance of the blades. For example, shrouds have been attached to the impeller blades by casting large impeller blades which are thick enough to locally receive a screw to attach the shroud. This type of attachment requires a relief hole through which the screw is inserted. The relief hole requires close tolerances, which can be burdensome and costly to the manufacturing process. This method generally only works for large impellers and is not desirable for a small thin impeller, as implemented in a permanent magnet motor/generator or a small gas turbine engine.
Accordingly, it is desirable to provide a method of attaching a shroud to a small impeller in a manner in which manufacturing costs are minimized and part quality and strength are enhanced.
The present invention overcomes the above-referenced shortcomings of prior art shroud/impeller assemblies by providing a shroud which is attached to an impeller by an interference fit.
Specifically, the present invention improves upon the compressor/turbine permanent magnet motor/generator of U.S. Pat. No. 5,899,673, and the efficiency of the xe2x80x98673 invention by controlling fluid flow with a shroud attached to the impeller. To accomplish such an improvement, this invention incorporates: a housing including first and second stators positioned within the housing and having respective channels cooperating to define a substantially annular pathway within the housing; a shaft rotatably supported within the housing; a rotatable element mounted for rotation with the shaft and having impeller blades substantially within the pathway, the impeller blades having respective radially extending distal ends defining an outer impeller diameter; and a shroud surrounding the rotatable element and connected thereto by an interference fit with at least some of the distal ends, the shroud in transverse cross-section having one configuration at the interference fit and another configuration outwardly thereof, whereby the other configuration cooperates with the channels of the first and second stators to define a helical pathway around the shroud.
Therefore, it is an object of the present invention is to attach the shroud to the impeller with an interference fit. This may be accomplished by either heating and cooling the shroud to be placed in juxtaposition to the impeller blades, or, alternatively, by forcing the shroud over the impeller blades.
Additionally, it is an object that the shroud in transverse cross-section may comprise a rounded portion which engages flowing fluid to encourage smooth fluid flow and discourage separation of the fluid from the shroud.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.